In the earlier part of this century, substantial quantities of anthracite coal were mined and processed in breakers to produce coal of various sizes. The coal was separated from mining refuse, and huge mounds of tailings were produced. Such tailings are currently being produced from active mining operations. Mine tailings are known to contain varying amounts of coal and refuse. For instance, a typical mound may contain 20-40% coal. Because of the steep rises in energy costs which have occurred within the last decade, it is now economically feasible to process these tailings to separate the coal from the refuse.
Processes are known for separating coal from refuse. One such process, which is practiced widely in this country, is the so-called heavy media cyclone separation process. In this process, separation takes place in a cyclonic separator which creates centrifugal forces which operate in conjunction with a slurry of a predetermined specific gravity to achieve separation.
The specific gravity of the medium is controlled by adding finely-pulverized magnetite ore to screened tailings to form a feedstock slurry which is admitted to the cyclonic separator. Utilizing known principles, the separator produces an overflow which is coal-rich and an underflow which is rich in refuse. The coal-rich overflow is dewatered and is rinsed with fresh water to produce a clean coal output and a magnetite-rich underflow slurry. The underflow slurry from the cyclonic separator is also dewatered and rinsed with fresh water to produce a refuse-rich reject material and a magnetite-rich underflow. The magnetite-rich underflow from both the coal-rich and refuse-rich dewatering screens are then passed through magnetic separators which separate the magnetite ore from its water carrier. The ore-less water than flows into a sump, and the separated ore is returned to a sump upstream of the cyclonic separator for admixing with more tailings and recirculation to the cyclonic separator.
While the thus-described prior art heavy media process functions satisfactorily, it has certain disadvantages. For instance, although the magnetic separators are intended to prevent losses of magnetite, substantial losses of magnetite result from the inherent limitations of the magnetic separators. Because of these limitations, it is not uncommon for magnetite ore losses to average 21/2 pounds per ton of raw input. Considering the fact that hundreds of tons of raw input per day can be processed in a typical plant, magnetite losses at current prices can run as high as $72,000/yr., based on 100 tons of input per hour. Thus, it should be apparent that magnetite-enriched heavy medium cyclonic separation techniques are not economical.
Other disadvantages of the magnetite-enriched heavy medium cyclonic separation process include the requirement of a substantial amount of fresh water, a significant capital investment for the magnetic separators, pumps, piping, and ancillary equipment, and a substantial ongoing outlay for operating and maintaining the equipment.
A magnetite-enriched heavy medium cyclonic separation process is disclosed in U.S. Pat. No. 2,726,763. Coal separation processes which use other types of medium are disclosed in U.S. Pat. Nos. 2,701,641; 2,649,963; 2,726,763; 2,860,252; 3,031,074; 2,819,795; 4,203,831; and 4,252,639. Separating cyclones are disclosed in one or more of the preceding patents and the following U.S. Pat. Nos. 2,724,503; 3,353,673; 4,164,467; 3,887,456; 4,175,036; 4,226,708; 3,379,308; and 3,902,601. For further information relating to coal-refuse separation processes, reference is hereby made to the disclosure contained in Chapter 10, entitled Wet Concentration of Fine Coal, of the book entitled Coal Preparation by Seeley W. Mudd published in 1968.